Image sensor and control method thereof, and image capturing apparatus

ABSTRACT

An image sensor comprises: a plurality of pixels; a plurality of column output lines; and a control unit configured to control a signal to be output from pixels selected from the plurality of pixels to the plurality of column output lines, and each of the plurality of pixels includes: a photoelectric conversion portion; a floating diffusion portion for holding charge transferred from the photoelectric conversion portion; and an addition portion to add capacitance to the floating diffusion portion. The control unit controls to add the capacitance to the floating diffusion portion in a case where signals are simultaneously output to the same column line from the selected pixels.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a technique of mixing signals read outfrom a plurality of pixels from an image sensor used in an imagecapturing apparatus, such as a camera, and outputting a mixed signal.

Description of the Related Art

Conventionally, an image capturing apparatus capable of shooting animage at high frame rate by mixing pixel signals and reading out themixed signals at high speed from an image sensor has been realized.Further, various counter measures to cope with problems that arises whenmixing pixel signals have been proposed.

For example, Japanese Patent Laid-Open No. 2010-259027 proposes that, ina case where signals from a plurality of pixels arranged in the samecolumn are simultaneously read out to a vertical output line to mix thepixel signals, a current value on the vertical output line is controlledto an optimum value in accordance with the number of pixels to be mixed.More specifically, the current on the vertical output line is increasedin a case where the pixel signals are mixed comparing to a case wherethe pixel signals are not mixed. Further, as the number of pixels to bemixed is increased, the current on the vertical output line isincreased, thereby expanding the optimum range for mixing pixel signals.The reason for changing the current in this manner is that, in a casewhere an amplitude of a pixel signal, in other words, of a signal on thevertical output line is large and a difference between pixel signals tobe mixed is large, circuit current of the pixel producing a largersignal decreases, which prevents the pixel signals from being mixedcorrectly, and thus it is necessary to compensate for the shortage ofcurrent in the vertical output line.

However, in order to optimally mix pixel signals as disclosed inJapanese Patent Laid-Open No. 2010-259027, energy consumption greatlyincreases.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and is to optimally mix pixel signals on a column output linewithout increasing energy consumption of an image sensor.

Further, the present invention is to mix pixel signals on the columnoutput line without deteriorating an S/N ratio.

According to the present invention, provided is an image sensorcomprising: a plurality of pixels; a plurality of column output lines;and a control unit configured to control a signal to be output frompixels selected from the plurality of pixels to the plurality of columnoutput lines, wherein each of the plurality of pixels includes: aphotoelectric conversion portion; a floating diffusion portion forholding charge transferred from the photoelectric conversion portion;and an addition portion to add capacitance to the floating diffusionportion, and wherein the control unit controls to add the capacitance tothe floating diffusion portion in a case where signals aresimultaneously output to the same column line from the selected pixels.

Further, according to the present invention, provided is the imagecapturing apparatus comprising: an image sensor disclosed above; and aprocessing unit configured to process a signal output from the imagesensor.

Furthermore, according to the present invention, provided is a controlmethod of an image sensor having a plurality of pixels, a plurality ofcolumn output lines, and a control unit configured to control a signalto be output from pixels selected from the plurality of pixels to theplurality of column output lines, wherein each of the plurality ofpixels includes a photoelectric conversion portion, a floating diffusionportion for holding charge transferred from the photoelectric conversionportion, and an addition portion to add capacitance to the floatingdiffusion portion, the method comprising: adding the capacitance to thefloating diffusion portion by the addition portion in a case wheresignals are simultaneously output to the same column output line fromthe selected pixels.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the description, serve to explain the principles of theinvention.

FIG. 1 is a diagram schematically illustrating a configuration of animage sensor according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating a configuration of a pixel portion anda readout unit of the image sensor according to the embodiment;

FIGS. 3A to 3C are timing charts for explaining readout operations of animage capturing apparatus according to the embodiment;

FIGS. 4A and 4B are diagrams for explaining change in voltage at thetime of mixing pixel signals in the image capturing apparatus accordingto the embodiment;

FIGS. 5A and 5B are diagrams showing image shooting conditions andsettings in the image capturing apparatus according to the embodiment;

FIG. 6 is a diagram illustrating a configuration of the pixel portion ofthe image sensor according to a modification of the embodiment; and

FIGS. 7A and 7B are diagrams showing image shooting conditions andsettings in the image capturing apparatus according the modification ofthe embodiment.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention will be described indetail in accordance with the accompanying drawings.

FIG. 1 is a diagram schematically illustrating the configuration of theimage sensor used in an image capturing apparatus according to anembodiment of the present invention. In FIG. 1, an image sensor 100includes a pixel portion 101, a vertical scanning unit 102, a readoutunit 103, a horizontal scanning unit 104, and a differential output unit110. The pixel portion 101 has a plurality of pixels 200, each having aconfiguration that will be explained later, arranged in a matrix, andreceives an optical image formed by an optical system (not shown).

The image sensor 100 may be provided with a timing generator or thelike, for providing a timing signal to each circuit described above.

FIG. 2 is a diagram illustrating a configuration of pixels of two rowsin one column out of the pixels 200 forming the pixel portion 101, andthe readout unit 103 for a column.

A photodiode (PD) 201 generates charge (referred to as “light charge”,hereinafter) by performing photoelectric conversion on light enteringvia the optical system (not shown). A transfer MOS transistor 202 iscontrolled by a Ptx(n) signal (n is a variable indicating a row number),and transfers the charge generated by the PD 201 to the floatingdiffusion portion (FD) 208 when it is on. The transferred light chargeis temporarily held in the FD 208.

A reset MOS transistor 203 is controlled by a Pres(n) signal, and whenit is turned on, the FD 208 is reset by a source voltage VDD. Further,by truing on the reset MOS transistor and the transfer MOS transistor202 simultaneously, it is possible to reset the PD 201.

The charge held in the FD 208 is output to the column output line 210via an amplification MOS transistor 204 when a selection MOS transistor205 controlled by a signal Psel(n) is turned on. The amplification MOStransistor 204 functions as a source follower amp when connected to avertical output line load 211 via the selection MOS transistor 205.

Further, a capacitor 206 is connected to the FD 208 via a FD additionswitch 207 driven by a Pfd(n) signal. By turning on the FD additionswitch 207, the capacitor 206 is added and the capacitance of a floatingdiffusion portion is increased; as a result, it is possible to reduce anamplitude of a signal output from the amplification MOS transistor 204.

The Ptx(n) signal, Pres(n) signal, Psel(n) signal, and Pfd(n) signal arerespectively driven by the vertical scanning unit 102 shown in FIG. 1.

A signal output to the column output line 210 is input to the readoutunit 103, and amplified by an amplifier 212. At this time, an output(noise signal) from the pixels 200, read out as will be explained later,right after the reset operation is ended is stored in a capacitor 216via a MOS switch 214 which is controlled by a PTN pulse. Further, asignal obtained as a result of photoelectric conversion of an incidentlight (referred to as “light signal”, hereinafter) is held in acapacitor 215 via a MOS switch 213 which is controlled by a PTS pulse.The noise signal and the light signal read out to the capacitors 216 and215, respectively, of the readout unit 103 as described above aresequentially selected by a ph signal which is controlled by thehorizontal scanning unit 104 on a column-by-column basis, and a signalobtained by taking the difference between the noise signal and the lightsignal by the differential output unit 110 is output.

The image capturing apparatus applies known processes, including imagedata generation for recording, to the signal output from the imagesensor having the above configuration in a signal processing unit (notshown).

Next, driving method of the image sensor 100 having the aboveconfiguration will be explained with reference to timing charts shown inFIGS. 3A to 3C. Here, a case where the pixels 200 in the ith row and thei+1th row are read out from among the pixels 200 arranged in a matrixwill be explained. In FIGS. 3A to 3C, the abscissa indicates time.Further, the row number to which each signal is applied is shown inparentheses after each signal name.

FIG. 3A shows an operation for independently reading out signals fromthe pixels in the ith row and the i+1th row. As shown in FIG. 3A, a HDsignal changes from a low level to a high level each time the signalsare read out from pixels 200 of one row (i.e., for 1 horizontal period).Further, it is assumed that when each control signal is in a high level,a corresponding transistor or transistors turn on. For the sake ofsimplicity of explanation, it is assumed that charge accumulation isperformed in the PD 201 of each pixel 200 before time T1, and theexplanation after time T1 is given below.

In the control shown in FIG. 3A, the Pfd(i) and Pfd(i+1) signals arealways in a low level, and thus the FD addition switches 207 are keptoff so as not to increase the capacitance of the floating diffusion (FDcapacitance is kept to a normal capacitance).

First, at time T2, the HD signal changes from a low level to a highlevel and the Psel(i) signal becomes a high level, thereby the ith rowis selected and the pixels 200 in the ith row are connected to thecolumn output lines 210. At this time, the Pres(i) is in a high level,and the FDs 208 of the pixels 200 in the ith row are reset to the sourcevoltage VDD.

Next, during a period between times T3 and T4, the PTS pulse and the PTNpulse become a high level, which turns on the MOS switches 213 and 214,and the capacitors 215 and 216 are reset. At time T5, the Pres(i) signalbecomes a low level, thereby the reset operation of the FDs 208 ends.

Thereafter, during a period between times T6 and T7, the PTN pulsebecomes a high level again and the MOS switches 214 are turned on, andan output (noise signal) from the pixels 200 in the ith row after thereset operation is ended is held in the capacitors 216. Next, during aperiod between times T8 and T9, the PTX(i) signal becomes a high level,and light change is transferred from the PDs 201 to the FDs 208 in allthe pixels 200 in the selected row. After that, during a period betweentimes T9 and T10, the PTS pulse becomes a high level again, which turnson the MOS switches 213, and an output (light signal+noise signal) fromthe pixels 200 in the ith row is held in the capacitors 215.

Thereafter, during a period between times T10 and T11, the lightsignal+noise signal held in the capacitors 215 and the noise signal heldin the capacitors 216 are transferred to the differential output unit110 provided in the downstream on a column-by-column basis by drivingthe ph pulse by the horizontal scanning unit 104, and a light signal,which is a difference between the transferred signals, is output.

At time T11, the Pres(i) signal becomes a high level, and at T12, the HDsignal changes from a low level to a high level again. At time T12 andon, the Psel(i) signal becomes a low level and the Psel(i+1) signalbecomes a high level so as to select the pixels 200 in the next row(here, the i+1th row), and signals of the pixels 200 in the i+1th roware read out in the same manner as the pixels 200 in the ith row. Byrepeating the aforesaid readout operation on a row-by-row basis, signalsare read out from all the pixels 200 of one frame, and thus pixelsignals of one frame representing a shot image of a subject can beobtained.

FIG. 3B shows a driving method for mixing signals from the two pixels200 in the vertical direction (column direction) by simultaneouslyconnecting the pixels 200 of two rows, the ith and i+1th rows, to thecolumn output lines 210, and outputting the resultant signals.

The HD signal changes from a low level to a high level each time signalsfrom the pixels 200 in the two rows (here, the ith row and the i+1throw) to be mixed are read out (i.e., for 1 horizontal period). Further,it is assumed that when each control signal is in a high level, acorresponding transistor or transistors turn on. Here, similarly to FIG.3A, it is assumed that charge accumulation is performed in the PD 201 ofeach pixel 200 before time T1, for the sake of simplicity ofexplanation, and the explanation after time T1 is given below.

It should be noted that in the control in FIG. 3B, the Pfd(i) andPfd(i+1) signals are also always in a low level, and thus the FDaddition switches 207 are kept off so as not to increase the FDcapacitance (i.e., kept to a normal capacitance).

First at time T2, the HD signal changes from a low level to a high leveland the Psel(i) and Psel(i+1) signals become a high level, thereby thepixels 200 in the ith and i+1th rows are connected to the column outputlines 210. At this time, the Pres(i) and Pres(i+1) signals are in a highlevel, and thus the FDs 208 of the pixels 200 in the ith and i+1th rowsare reset to the source voltage VDD.

Next, during a period between times T3 and T4, the PTS pulse and the PTNpulse become a high level, which turns on the MOS switches 213 and 214,and the capacitors 215 and 216 are reset. At time T5, the Pres(i) andPres(i+1) signals become a low level, thereby the reset operation of theFDs 208 ends.

Thereafter, during a period between times T6 and T7, the PTN pulsebecomes a high level again and the MOS switches 214 are turned on, andan output (noise signal) from the pixels 200 in the ith and i+1th rowsafter the reset operation is ended is held in the capacitors 216. Next,during a period between times T8 and T9, the Ptx(i) and Ptx(i+1) signalsbecome a high level, and light charge is transferred from the PDs 201 tothe FDs 208 in all the pixels 200 in the selected rows (here, the ithand i+1th rows). After that, during a period between times T9 and T10,the PTS pulse becomes a high level again, which turns on the MOSswitches 213, an output (light signal and noise signal) from the pixels200 in the ith and i+1th rows is held in the capacitors 215.

Thereafter, during a period between times T10 and T11, the lightsignal+noise signal held in the capacitors 215 and the noise signal heldin the capacitors 216 are transferred to the differential output unit110 provided in the downstream on a column-by-column basis by operatingthe Ph pulse by the horizontal scanning unit 104, and a light signal,which is a difference between the transferred signals, is output.

At time T11, the Pres(i) and Pres(i+1) signals become a high level, andat time T12, the HD signal changes from a low level to a high levelagain. After time T12 and on, the Psel(i) and Psel(i+1) signals become alow level, and the Psel(i+2) and Psel(1+3) signals become a high levelso as to select the pixels 200 in the next two rows (here, the i+2th andi+3th rows). Then, signals of the pixels 200 in the next two rows arereadout in the same manner as the pixels 200 in the ith and i+1th rows.By repeating the mixing of the pixel signals and reading of the mixedsignals, signals are read out from all of the pixels 200 of one frame,and thus it is possible to obtain a frame of pixel signals each madefrom signals of two pixels mixed in the vertical direction (columndirection) representing a shot image of a subject.

Similarly to FIG. 3B, FIG. 3C shows a driving method for mixing signalsfrom the two pixels 200 in the vertical direction (column direction) bysimultaneously connecting the pixels 200 of two rows, the ith and i+1throws, to the column output lines 210, and outputting the resultantsignals. A difference between FIGS. 3B and 3C is that the Pfd(i) signaland the Pfd(i+1) signal are kept in a high level, namely, the FDaddition switches 207 are on, thereby the FD capacitance is increased.Operation timing of other signals are the same as those shown in FIG.3B, and thus the explanation thereof is omitted.

FIGS. 4A and 4B show graphs of voltage Vo of a column output line 210with respect to voltage (FD voltage) Vfd of a floating diffusionportion. The abscissa indicates the FD voltage Vfd, and the ordinateindicates the voltage Vo of the column output line 210. In FIGS. 4A and4B, an explanation will be given of under assumption that in a casewhere signals are mixed and read out by two pixels 200 when the pixels200 receive light from a high-contrast subject, one of the two pixels200 receives light and the other pixel 200 does not receive light atall.

A line 401 shows relationship between the FD voltage Vfd and the voltageVo of the column output line 210 in a case where only the pixel 200which receives light is connected to the column output line 210. A curve402 shows relationship between the FD voltage Vfd of the pixel 200 whichreceives light and the voltage Vo of the column output line 210 in acase where the pixel 200 which receives light and the pixel 200 whichdoes not receive light are simultaneously connected to the column outputline 210 to mix the pixel signals.

FIG. 4A shows a case in which signals of two pixels 200 in the verticaldirection (column direction) are mixed and outputted by the operationexplained with reference to FIG. 3B. As shown by the line 401, in thepixel 200 that receives light, the voltage Vo of the column output line210 linearly changes within a range (ΔV1) between Vfdres which is areset voltage of the FD voltage Vfd and Vpdsatl which is the FD voltageVfd when the PD is saturated and charge is transferred.

By contrast, the curve 402 showing a case in which pixel signals aremixed is ideally supposed to be a straight line having ½ of the tilt ofthe line 401 if signals from the two pixels 200 are properly mixed.However, the curve 402 starts to bent around a point where the FDvoltage Vfd of the pixel 200 which receives light changes by ΔV2 fromthe FD reset voltage Vfdres as an infection point, and eventually stopsto change as described in Japanese Patent Application Laid-Open No.2010-259027. In other words, in the bent part of the curve 402, thesignals are not mixed properly.

Accordingly, in order to properly mix signals from the two pixels 200,it is desirable to drive within a range, as shown in FIG. 4B, wherelinearly is sufficiently secured. More specifically, the range isbetween the FD reset voltage Vfdres and a FD voltage when charge istransferred from a saturated PD, where the FD voltage Vfd of the pixel200 changes by Vpdsat3 (ΔV3).

As explained with reference to FIG. 3C, in a case where two pixelsignals are to be mixed, the Pfd(n) signal is set to a high level tokeep the FD addition switch 207 on, thereby it is possible to increasethe FD capacitance. By doing so, it is possible to reduce a changingrange of the FD voltage to a range of ΔV3 shown in FIG. 4B.

Meanwhile, an image capturing apparatus such as a regular camera or thelike generally has a function for changing a gain (ISO sensitivitysetting) of an amplification unit provided downstream in accordance witha luminance of a subject to be shot. For example, a control is made suchthat, in a case of shooting a bright subject, a low sensitivity, such asISO100, is set and the gain is decreased, whereas, in a case of shootinga dark subject, a high sensitivity, such as ISO1600, is set and the gainis increased. If the output range of a voltage after amplified by thegain in the amplification unit is constant independent of the ISOsensitivity, the voltage range (FD voltage range) of a floatingdiffusion portion is large when a low sensitivity is set, and the FDvoltage range is small when a high sensitivity is set.

In a case where the ISO sensitivity is changed when the signals of twopixels 200 in the vertical direction (column direction) are mixed by theoperation shown in FIG. 3C, the FD voltage range is as shown in FIG. 4B.For example, the FD voltage ranges for ISO100, ISO200 and ISO400 areΔV3, ΔV4 and ΔV5, respectively, where ΔV3=2×ΔV4=4×ΔV5. It is known thatthe light charge transferred from the PD decreases as the FD capacitanceincreases, and an S/N ratio deteriorates especially when high ISOsensitivity is set.

Therefore, it is possible to prevent the S/N ratio from deterioratingwhen high ISO sensitivity is set by limiting the operation forincreasing the FD capacitance as shown in FIG. 3C to be performed towhen low ISO sensitivity is set, and by performing the operation for notincreasing the FD capacitance as shown in FIG. 3B when other ISOsensitivity is set. Although the FD voltage range is widened when thehigh ISO sensitivity is set, it is possible to properly mix signalsoutput from the two pixels in the vertical direction (column direction)within a range where linearity of the voltage Vo of the column outputline 210 can be secured.

Examples of combinations of the ISO sensitivity, FD capacitance, andgain of the amplification unit arranged downstream are shown in FIGS. 5Aand 5B. In FIGS. 5A and 5B, “ISO” indicates ISO sensitivity, “Tfd”indicates on/off of the FD addition switch 207 driven by the Pfd signal,and “Cfd” indicates a ratio of total FD capacitance. In a case where theFD addition switch 207 is off (i.e., Tfd=OFF), the capacitor 206 is notconnected and thus the total FD capacitance is ×1, and in a case wherethe FD addition switch 207 is on (i.e., Tfd=ON), the capacitor 206 isconnected and thus the total FD capacitance becomes ×2. “Vfdr” indicatesa ratio of the FD voltage range at the time of shooting a subject, and“gain” indicates a gain of the amplification unit arranged downstream.Here, the capacitance of the FD 208 and the capacitance of the capacitor206 are assumed to be the same.

For all of the combinations shown in FIG. 5A, the FD addition switch 207is off (Tfd=OFF) for every ISO sensitivity. These settings are used inan image shooting operation in a case where it is determined that thepixel signals are not mixed since the FD voltage range especially in thelow ISO sensitivity is large (for example, a moving image shooting at alow frame rate, a still image shooting, and so forth). For example, theratio Vfdr of the FD voltage range in ISO100 is 1 and the gain at thattime is 1, by contrast, the ratio Vfdr of the FD voltage range in ISO200is ½ and the gain at that time is 2.

In the operation shown in FIG. 5B, the FD addition switch 207 is turnedon (Tfd=ON) only in a case where ISO100 is set in order to double the FDcapacitance and halve the FD voltage range. In other ISO sensitivities,the FD addition switch 207 is set to off (Tfd=OFF). These settings areused in an image shooting operation in a case where it is determinedthat the pixel signals are to be mixed (for example, a moving imageshooting at a high frame rate in which all pixel readout is notpossible, and so forth). The ratio Vfdr of the FD voltage range inISO100 is set to ½ and the gain at that time is set to ×2. For other ISOsensitivities, the same operation as those of FIG. 5A is performed.

In a case where pixel signals are to be mixed, as shown in FIG. 5B, theset ISO sensitivity is determined, and in the low ISO sensitivity, it ispossible to optimally mix pixel signals by increasing the FD capacitanceand amplifying a signal corresponding to a decreased portion of the FDvoltage range. Further, since the FD capacitance is not increased in theother ISO sensitivities (high ISO sensitivities), it is possible toobtain pixel signals of a high S/N ratio.

<Modification>

FIG. 6 is a diagram illustrating an equivalent circuit diagram of apixel 300 of the image sensor according to a modification of theembodiment of the present invention. In FIG. 6, the same constituents asthose shown in FIG. 2 are referred to by the same reference numerals.The difference between the pixel 300 shown FIG. 6 and the pixel 200shown in FIG. 2 is that a second capacitor 310 is connected to the FD208 via a second FD addition switch 309 which is driven by a Pfd2signal. By turning on the second FD addition switch 309, it is possibleto increase a FD capacitance, and to decrease an amplitude of a signaloutput from the pixel 300. Note that the Pfd2 signal is also driven bythe vertical scanning unit 102 shown in FIG. 1.

The operating method of the image sensor 100 comprising the pixels 300having the structure as shown in FIG. 6 is explained for each ISOsensitivity with reference to FIGS. 7A and 7B. In the table shown inFIGS. 7A and 7B, “Tfd2” for indicating ON/OFF of the second FD additionswitch 309 driven by the Pfd2 signal is added. Here, the capacitance ofthe second capacitor 310 is twice as large as the capacitance of the FD208 and the capacitor 206.

When both of the FD addition switch 207 and the second FD additionswitch 309 are off (Tfd=OFF, Tfd2=OFF), the capacitors 206 and 310 arenot connected, and the total FD capacitance is ×1. When the FD additionswitch 207 is on (Tfd=ON) and the second FD addition switch 309 is off(Tfd2=OFF), the capacitor 206 is connected but the capacitor 310 is notconnected, therefore the total FD capacitance becomes ×2. Further, whenboth of the FD addition switch 207 and the second FD addition switch 309are on (Tfd=ON, Tfd2=ON), the capacitors 206 and 310 are connected, andthe total FD capacitance becomes ×4. “Vfdr” indicates a ratio of the FDvoltage range when shooting a subject, and “gain” indicates a gain ofthe amplification unit arranged downstream.

In the combinations shown in FIG. 7A, the FD addition switch 207 and thesecond FD addition switch 309 are off (Tfd=OFF, Tfd2=OFF) for every ISOsensitivity. This operation is used in an image shooting in a case whereit is determined that pixel signals are not mixed since the FD voltagerange is large especially in a low ISO sensitivity (for example, amoving image shooting at low frame rate, a still image shooting, and soforth). For example, in ISO100, the ratio Vfdr of the FD voltage rangeis 1 and the gain at that time is ×1, and in ISO200, the ratio Vfdr ofthe FD voltage range is ½ and the gain at that time is ×2.

In the operation shown in FIG. 7B, both of the FD addition switch 207and the second FD addition switch 309 are turned on (Tfd=ON, Tfd2=ON) ina case of ISO100, thereby the FD capacitance is increased by ×4, and theratio Vfdr of the FD voltage range is set to ¼. Further, in a case ofISO200, the FD addition switch 207 is turned on (Tfd=ON) and the secondFD addition switch 309 is turned off (Tfd2=OFF), thereby the FDcapacitance is increased by ×2, and the ratio Vfdr of the FD voltagerange is set to ¼. In the other ISO sensitivities, both of the FDaddition switch 207 and the second FD addition switch 309 are turned off(Tfd=OFF, Tfd2=OFF).

The operation as shown in FIG. 7B is performed in an image shooting in acase where it is determined that pixel signals are to be mixed (forexample, a moving image sensing at a high frame rate in which all pixelreadout is not possible, and so forth). Especially, this operation isvery effective in an image sensor having an especially small FDcapacitance in a case where an extra FD capacitance is not added. Inthis example, in ISO100, the ratio Vfdr of the FD voltage range is ¼ andthe gain at that time is ×4. Further, in ISO200, the ratio Vfdr of theFD voltage range is ¼ and the gain at that time is ×4, and in other ISOsensitivities, the same operation as that of FIG. 7A is performed.

By operating an image sensor 100 as shown in FIG. 7B, in a case wherepixel signals are mixed in the image sensor 100 having a particularlysmall FD capacitance, the FD capacitance is increased when a low ISOsensitivity is set and a portion corresponding to a reduced amount ofthe FD voltage range is amplified in downstream, it is possible tooptimally mix pixel signals. Further, since the FD capacitance is notincreased in other ISO sensitivities (high ISO sensitivities), it ispossible to obtain a pixel signal of a high S/N ratio.

Note that in the above embodiment and modification, pixels whose pixelsignals are added are two successive pixels as shown in FIG. 2, however,the present invention is not limited thereto. For example, the presentinvention is applicable to a case where signals from two or more samecolor pixels in the Bayer arrangement are mixed.

Further, the selection MOS transistor 205 is used in FIGS. 2 and 6,however, the present invention is not limited thereto. For example,other circuit structures capable of activating the amplification MOStransistor 204 to output to the column output line 210 may be used.

Furthermore, in FIGS. 2 and 6, the sources of addition switch 207 andthe second FD addition switch 309 are connected to the capacitors 206and 310 whose other terminals are grounded, however, parasiticcapacitance may be used instead if the parasitic capacitance is largeenough to realize the present invention.

Further, as shown in FIG. 6 for explaining the modification, by furtheradding a FD addition switch and a capacitor and appropriatelycontrolling on/off of a FD addition switch, it is possible to mix pixelsignals more appropriately.

Further, in the above embodiment and modification, it is assumed thatpixel signals are mixed and read out, and the FD capacitance isincreased in a case of low ISO sensitivity (a predetermined capacitanceor less). However, it is possible to configure the pixel 200 or 300 suchthat the FD capacitance is increased in a case where pixel signals areto be mixed and read out regardless of the ISO sensitivity.

Further, in the above embodiment and modification, whether or not toincrease the FD capacitance is controlled by taking the ISO sensitivityas a condition for the operation, however, the present invention is notlimited to the ISO sensitivity, and can be controlled on the basis ofthe brightness of a subject. In this case, when the photometry value islarger than a predetermined value and the variation of voltage acrossthe FD 208 does not fall within the range ΔV3 in FIG. 4B, it may beconsidered to control the FD capacitance being increased.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-109430, filed on May 27, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image sensor comprising: a plurality of pixelseach including: a photoelectric conversion portion; a floating diffusionportion which holds charge transferred from the photoelectric conversionportion; an addition capacitor; and a connection switch which connectsthe additional capacitor to the floating diffusion portion; a pluralityof column output lines; and a controller which controls the connectionswitch to connect the additional capacitor to the floating diffusionportion in a case where signals are simultaneously output to the samecolumn output line of the plurality of column output lines from pixelsselected from the plurality of pixels.
 2. The image sensor according toclaim 1, wherein the connection switch is provided between the floatingdiffusion potion and the additional capacitor.
 3. The image sensoraccording to claim 1, wherein the controller controls the connectionswitch to connect the additional capacitor to the floating diffusionportion in a case where a predetermined condition is satisfied.
 4. Theimage sensor according to claim 3, wherein the predetermined conditionincludes a condition which a set sensitivity is equal to or less than apredetermined sensitivity.
 5. The image sensor according to claim 3,wherein the predetermined condition includes a condition in which abrightness of a subject is brighter than a predetermined brightness. 6.The image sensor according to claim 1, wherein the each of the pixelsincludes a plurality of additional capacitors and a plurality ofconnection switches.
 7. The image sensor according to claim 6, whereinthe controller controls to connect more additional capacitors to thefloating diffusion portion in a case where a brightness of a subject isbrighter than a predetermined brightness than in a case where thesubject is darker than the predetermined brightness.
 8. The image sensoraccording to claim 1, wherein the controller controls to connect theadditional capacitor to the floating diffusion portion by the connectionswitch before transferring charge from the photoelectric cot versionportion to the floating diffusion portion.
 9. An image capturingapparatus comprising: an image sensor comprising: a plurality of pixelseach including: a photoelectric conversion portion; a floating diffusionportion which holds charge transferred from the photoelectric conversionportion; an addition capacitor; and a connection switch which connectsthe additional capacitor to the floating diffusion portion; a pluralityof column output lines; and a controller which controls the connectionswitch to connect the additional capacitor to the floating diffusionportion in a case where signals are simultaneously output to the samecolumn output line of the plurality of column output lines from pixelsselected from the plurality of pixels; and a processor which processes asignal output from the image sensor.
 10. The image capturing apparatusaccording to claim 9, further comprising an amplification an amplifierwhich amplifies the signal output from the image sensor, wherein theamplifier changes its gain in accordance with increased capacitance ofthe floating diffusion portion and the additional capacitor in a casewhere the connection switch connects the additional capacitor to thefloating diffusion portion under control of the controller.
 11. Theimage capturing apparatus according to claim 9, wherein the imagecapturing apparatus is capable of performing a still image shooting anda moving image shooting, and in a case where a moving image shooting isperformed at a frame rate at which all pixel readout is impossible, thecontrol unit controls to simultaneously select two or more pixels andoutput signals from the selected pixels.
 12. A control method of animage sensor having a plurality of column output lines and a pluralityof pixels including a photoelectric conversion portion, a floatingdiffusion portion which holds charge transferred from the photoelectricconversion portion, an addition capacitor, and a connection switch whichconnects the additional capacitor the floating diffusion portion, saidmethod comprising: connecting the additional capacitor to the floatingdiffusion portion by the connection switch in a case where signals aresimultaneously output to the same column output line of the plurality ofcolumn output lines from pixels selected from the plurality of pixels.13. The method according to claim 12, further comprising judging whetheror not a set sensitivity is equal to or less than a predeterminedsensitivity, wherein the connecting step connects the additionalcapacitor to the floating diffusion portion by the connection switch ina case where the set sensitivity is equal to or less than thepredetermined sensitivity.
 14. The method according to claim 12, furthercomprising judging whether or not a brightness of a subject is equal toor brighter than a predetermined brightness, wherein the connecting stepconnects the additional capacitor to the floating diffusion portion bythe connection switch in a case where the subject is equal to orbrighter than the predetermined brightness.